As such fans have been designed, the blades are relatively wide, to provide adequate air flow at low speed, but the blade material is extremely thin, e.g., 0.015 inch, to permit rapid deflection of the blades at higher speeds to minimize power consumption and noise when air flow requirements are met in large measure by vehicle motion. Unfortunately, the natural frequency of such blades sometimes falls close to or within the firing frequency range of an engine at idle, resulting in blade vibration of substantial amplitude. Under some circumstances this vibration can lead to fatigue and premature failure of the fan blades.
The vibration problem can be met by using heavier blade material and/or by reducing the blade width to increase the blades natural frequency. This expedient, however, results in a sacrifice of desirable blade deflection characteristics at high speed in a flexible blade fan.
It has also been suggested, as in U.S. Pat. Nos. 3,625,413 and 4,037,987 and in my prior copending application Ser. No. 843,565, filed Oct. 19, 1977, that the vibration problem be met by raising the blade natural frequency to a frequency which is above the normal idle speed of the engine.